This invention relates to acoustic well logging in general and more particularly to acoustic compressional wave well logging and acoustic shear wave well logging.
Acoustic well logging has been generally accepted as a method for obtaining information about subterranean formations surrounding wells or boreholes. The applications of acoustic well logging include the determination of formation lithology, density, and porosity, the conversion of seismic time sections to depth sections, the determination of elastic constants of subsurface materials, and the detection of fractures and provision of information about their orientation.
In acoustic well logging it is customary to measure the compressional wave or shear wave velocity of subterranean formations surrounding boreholes. Compressional waves are also known as P-waves. Shear waves are also known as S-waves. A conventional P-wave velocity logging system includes a cylindrical logging sonde suitable for suspension in a borehole liquid, a source connected to the sonde for generating P-waves in the borehole liquid and two detectors connected to the sonde and spaced apart from the P-wave source for detecting P-waves in the borehole liquid. A compressional wave in the borehole liquid generated by the source is refracted in the earth formation (the phrase "earth formation" will be used throughout this specification to denote any subterranean formation, and will not be used in a narrow sense to denote any particular type of subterranean formation) surrounding the borehole. It propagates through a portion of the formation and is refracted back into the borehole liquid and detected by two detectors spaced vertically apart from each other and from the P-wave source. The ratio of the distance between the two detectors to the time between the detections of the P-wave by the two detectors yields the P-wave velocity of the formation. The P-waves in the borehole liquid caused by refraction of P-waves back into the borehole may be called the P-wave arrival.
The compressional wave velocity of the earth formation surrounding a borehole frequently varies with radial distance from the borehole. Several factors can give rise to such an effect, including drilling damage to the formation, penetration of the formation adjacent to the borehole by borehole drilling fluids, and in the Arctic, melting of permafrost near the borehole.
The part of the formation that has been so damaged, penetrated, or melted is known as the invaded zone, and the remaining part of the formation which has not been so affected, the virgin formation. Thus, the borehole is surrounded by the invaded zone which in turn is surrounded by the virgin formation. The compressional wave velocity of the virgin formation is usually different from that of the invaded zone. It is well known that compressional wave velocity logging of the virgin formation will yield information helpful for determining the porosity, rock lithology and density of the virgin earth formation.
The conventional P-wave logging source is symmetrical about the logging sonde axis. The P-waves generated by a conventional symmetrical source do not penetrate deeply into the earth formation surrounding the borehole. The depth of penetration of the P-wave arrival depends on the distance or spacing between the P-wave source and the detectors: the greater the source-detector spacing, a greater part of the P-wave energy refracted back into the borehole and detected will have penetrated deeper. With the spacing of six to ten feet commonly used in conventional P-wave logging, most of the energy of the P-waves generated by the source and detected by the detectors frequently does not penetrate beyond the invaded zone and only a small part of the P-wave energy reaches the virgin formation. The P-waves that reach and travel in the virgin formation typically will have smaller amplitudes than will the P-waves that do not penetrate beyond the invaded zone so that their arrivals may be masked by the arrivals of the P-waves that do not penetrate beyond the invaded zone. Therefore, where the source-detector spacing does not exceed the conventional spacing of six to ten feet, it may not be possible to use a symmetrical conventional source to log the P-wave velocity of the virgin formation. The source-detector spacing may be increased to increase the penetration of the P-waves. Increasing the source-detector spacing will, however, reduce the signal strength of the P-wave arrival. The attenuation of the P-waves traveling in the formation increases with the distance they travel in the formation. Thus, if the source-detector spacing is increased, the P-wave arrivals detected will be weaker and the resulting P-wave log may have a poor signal-to-noise ratio. It is thus desirable to increase the penetration of the P-waves without increasing the source-to-detector spacing.
Asymmetric compressional wave sources have been developed for logging the shear wave velocity of an earth formation. In such asymmetric sources, the source generates in the borehole fluid a positive pressure wave in one direction and a simultaneous negative pressure wave in the opposite direction. The interference of the two pressure waves produces a shear wave which is refracted in the earth formation. This type of asymmetric source is disclosed by European patent application Ser. No. 31989 by Angona et al., U.S. Pat. No. 3,593,255 to White, issued July 13, 1971, and U.S. Pat. No. 4,207,961 to Kitsunezaki, issued June 17, 1980.
Angona et al. discloses a bender-type source which comprises two circular piezoelectric plates bonded together and attached to a logging sonde by their perimeters. When voltage is applied across the two piezoelectric plates, the center portion of the circular plates will vibrate to create a positive compressional wave in one direction and a simultaneous negative compressional wave in the opposite direction. The two compressional waves will interfere to produce a shear wave in the earth surrounding the borehole. The bender-type source disclosed by Angona et al., will have a limited frequency range. It is specified in Angona et al., that the apparatus disclosed therein is capable of generating an acoustic signal having frequency components in the range of about 1 to 6 kHz, a frequency range in which the amplitude of the shear waves generated and refracted in the formation will likely be significantly greater than that of the P-waves generated and refracted in the formation, and thus a frequency range too low for compressional wave logging in most formations.
White discloses an asymmetric source comprising two piezoelectric segments each in the shape of a half hollow cylinder. The two segments are assembled to form a split cylinder. The two segments have opposite polarization and electrical voltage is applied to each segment, causing one segment to expand radially and simultaneously causing the other segment to contract radially, thereby producing a positive compressional wave in one direction and a simultaneous negative compressional wave in the opposite direction. The two compressional waves will interfere to produce a shear wave in the earth formation adjacent to the borehole. Such shear wave propagates along the borehole and is detected by a pair of transducers positioned substantially directly above or beneath the piezoelectric segments of the source. The White apparatus "accentuates" the shear waves and virtually eliminates the faster-traveling compressional waves generated and detected thereby. White does not disclose or suggest apparatus generally suitable for compressional wave logging. Nor does White disclose or suggest any method suitable for acoustic velocity logging of the virgin formation surrounding a borehole.
In Kitsunezaki, coils mounted on a bobbin assembly are placed in the magnetic field of a permanent magnet and current is passed through the coils to drive the bobbin assembly. The movement of the bobbin assembly ejects a volume of water in one direction and simultaneously sucks in an equivalent volume of water in the opposite direction, thereby generating a positive compressional wave in one direction and a simultaneous negative compressional wave in the opposite direction. Kitsunezaki's asymmetric source, however, cannot be driven at high frequencies or with sufficient power required for compressional wave logging in most formations. Also it cannot operate at great depths or under great pressures.
In another type of asymmetric shear wave logging source, instead of coupling the source to the borehole wall through the medium of the borehole fluid, the source is either coupled directly to the borehole wall or through mechanical means such as mounting pads. Such shear wave logging sources are disclosed in U.S. Pat. No. 3,354,983 to Erickson et al., issued Nov. 28, 1967, and U.S. Pat. No. 3,949,352 to Vogel, issued April 6, 1976.